This invention relates to a dental investment material for providing inexpensively a dental investment mold free from cracks, crevices or breakage even when it is subjected to rapid burnout and to a dental casting mold formed by using such dental investment material. This invention further relates to a process for burning out such dental investment, in which not only occurrence of cracks, crevices and breakage can securely be prevented but also metal casting operation time can greatly be reduced.
in the process of producing dental castings such as crowns, bridges and inlays, an investment material in which a wax pattern thereof is invested is used. After the investment material is set, the wax is eliminated by melting out and following the burning out process to provide a mold with a cavity of the same shape as that of the wax pattern. One of the important characteristics required for such investment materials is a degree of thermal expansion to compensate for shrinkage of cast metal to some extent in solidifying. Unlike the ordinary metal castings, it is difficult in the production of dental castings to produce a model having a slightly larger size than the actual tooth so as to compensate for such casting shrinkage in solidifying.
As the conventional dental investment materials for forming dental casting molds, a gypsum-cristobalite system investment material prepared by admixing cristobalite to a calcined gypsum and a gypsum-quartz system investment material prepared by admixing quartz to a calcined gypsum are widely utilized.
By the way, such types of dental casting molds are of a hollow body obtained by the molding of an investment material using a wax tooth model, and heating, followed by wax elimination. When a casting mold is prepared using thus obtained mold, it is burned out at a prescribed burnout temperature of about 700.degree. C., and a molten alloy is casted therein. In this burnout process, it has conventionally been performed to heat the investment gradually from a low temperature such as about room temperature to a prescribed burnout temperature so as to prevent the investment from suffering from the occurrence of cracks, crevices or breakage. Accordingly, it takes a considerable time for such elevation of temperature. Further, the temperature of the furnace muse be lowered almost to normal temperature to start the next casting operation, requiring a great loss of time. Thus, the conventional investment materials suffer from extremely poor working efficiency, disadvantageously. Besides, since these investment materials cannot be subjected to rapid heating (to start heating at a high temperature or to heat at a high heating rate), they also suffer problems that they cannot cope with a case of emergency and that the life of furnace is shortened by the damage caused by the repeated heating/cooling cycle.
Details of such problems are described in Skinner's "The Science of Dental Materials", (1991) valued as a text of dental materials, as follows:
The rate at which the investment is heated is a factor in attaining a smooth surface on the casting. If heating is too rapid initially, the steam resulting from the elimination of the free water and water of crystallization may cause the walls of the mold to flake off as the steam emerges from the investment.
Too rapid heating may also cause cracking in the investment. In such a case, the outside layer of the investment becomes heated before the center portions. Consequently, the outside layer starts to expand thermally, resulting in compressive stress in the outside layer, counteracting tensile stress in the middle regions of the mold. Such a stress distribution causes the brittle investment to crack from the interior outwardly in the form of radial cracks. These cracks, in turn, will produce a casting with fins or spines similar to those shown in FIG. 23-7 of said Skinner's, "The Science of Dental Materials". This condition is especially likely to be present with a cristobalite investment. The comparatively low inversion temperature of the cristobalite, and the rapid rate of expansion during the inversion, make it especially important to heat the investment slowly.